The invention relates to an electrical contact surface suitable for formation of electrical switch contacts, electrical connectors, and the like. Typically, electrical connectors are made from a copper, or like, substrate coated with nickel overlayered with gold. For instance one typical conventional electrical connector is a copper surface coated with 100 microinches of nickel overcoated with 50 microinches of gold. Gold offers both low contact resistance and resistance to corrosion. However, gold is very expensive and adds greatly to the cost of the electrical contact surfaces.
According to the present invention, an electrical contact surface is provided which has current resistance that compares favorably to that of conventional gold contact surfaces. The low contact resistance of the electrical contact surface according to the invention is particularly applicable to heremetically sealed environments, but can also have sufficient corrosion resistance to be used in a wide variety of environments. The electrical contact surface according to the invention is produced much more cheaply than standard electrical connectors with gold coatings.
According to one aspect of the present invention, a method is provided for forming an electrical contact surface by immersing a substrate in a particular plating bath. The plating bath includes the nickel ion as the primary depositable cation and the anion may be chloride, sulfate, sulfamate, or mixtures of these materials in an amount between about 0.10-2 molar, with a level of about 0.5 molar being preferred. The bath may also include anion additives selected from the group consisting essentially of TiF.sub.6 --, ZrF.sub.6 --, HfF.sub.6 --, and TaF.sub.7 --. The anions are in an amount between about one gram per liter and the solubility limit of the particular anions. The bath temperature and pH conditions are controlled so as to effect electrolytic deposition of the nickel in a crystalline form on the substrate. The temperature may be maintained between 5.degree.-100.degree. C., with a preferred level of about 60.degree. C., while the pH is maintained between about 0.5-5, with a preferred level of about 1.5. Electrodeposition is not significantly affected by current density, within a wide range of current densities, the typical current densities being in the general area of 200 milliamperes per square centimeter. After electrolytic deposition, the substrate is removed from the bath.
The electrical contact surface that is produced according to the present invention has a matte finish, with asperities or nodules densely arranged on the surface. The nature of the nodules is such that a very high point contact pressure is created when contact is made, overcoming any oxidation or corrosion products which may be present. The nickel is in its normal crystalline form, and contains less than one percent of any other material. The contact resistance of the surface is low, e.g. on the order of 1.3 milliohms, and in any event usually less than 1.5 milliohms, which is the minimum contact resistance of conventional electrical connectors formed from a standard copper surface coated with 500 microinches of gold. Contact resistance is determined according to ASTM B667-80.
It is the primary object of the present invention to provide for the production of electrical contact surfaces having low contact resistance without the necessity of utilizing a gold coating. This and other objects of the invention will become clear from an inspection of a detailed description of the invention and from the appended claims.